Recently, digital radio communication has occupied the important position in the telecommunication field by virtue of its drastic improvement in transmission speed and quality. Meanwhile, radio communication, making use of a radio wave space as a public property, involves a fundamental drawback that interception might be by a third party when considered from a concealment point of view. Namely, there always exists a fear that the communication content be intercepted by and leaked of information to the third party.
Accordingly, the conventional radio communication is devised by encrypting information in such a manner that, should communication data be intercepted by a third party, information content could not be known to the third party. Encryption is now under study and applied in various fields. This is owing to the benefit of encryption that a constant level of security is to be secured without modifying the communication system.
However, information encryption involves a problem that information is comparatively easy to crack where the encryption code or its procedure becomes aware of. Particularly, in the existing situation that high-speed computers are generally in widespread dissemination, security is not to be secured without the use of a significantly complicated encryption process.
To cope with the problem encountered in such an encryption art, there is proposed a radio transmission scheme with high level of concealment based upon the notice to the physical feature of a propagation channel, that is, radio wave propagation medium (e.g. JP-A-2002-152191). According to this method, communication data is exchanged by taking into account the propagation channel characteristic shared between the particular radio stations. Because of the impossibility to receive or restore the data at other radio stations having different propagation channels, security can be obtained on the physical layer of radio communication. Meanwhile there is another art that a secret key is shared between particular radio stations in order to encrypt data based upon a propagation channel having a characteristic unique to those, thereby preventing the third party from eavesdropping (e.g. Motoki Horiike and three others, “A Scheme of Secret Key Agreement Based on the Random Fluctuation of Channel Characteristics in Land Mobile Radio”, Shingaku-giho, RCS2002-173, October 2002).
In those radio transmission schemes making use of the randomness over the radio transmission channel, the communication data requiring concealment can be prevented from being intercepted by raising the probability of error occurrences over the transmission channel upon an interception by a third party. Accordingly, communication is made possible with higher security by combining it with an information source key encryption technique in general utilization.
In the mobile communication system of cellular telephony or WLAN, the propagation channel characteristic between two radio stations, usually, is characterized by the spatial locations of radio stations. The parameters describing the propagation channel characteristic uses amplitude/phase, arrival wave directions, delay time, polarization and so on. In case the propagation channel between particular radio stations can be uniquely characterized by use of those parameters, communication could be realized with concealment by taking propagation parameters into consideration. The uniqueness can be considered enhanced furthermore by increasing the number of parameters and expressing the propagation channel characteristic by means of a multi-dimensional parameter.
However, where implementing radio communication while taking into consideration the propagation parameters estimated from the propagation channel between the radio stations, in case attempted to increase the number of parameters or improve the estimation accuracy of parameters for improving concealment, there arises a problem that higher accuracy is required for the hardware besides the increased amount of signal processing.
Meanwhile, in the case of producing a common key for use in encryption depending upon the propagation parameters, both the two radio stations are required to execute the process of propagation parameter estimation and key production. For example, assuming the communication between the base station and the terminal, there is a problematic need to improve the hardware accuracy of the communication terminal on a tendency toward upgraded functions, particularly, as to applications and interfaces, in order to increase signal processing amount and ensure estimation accuracy.